1. Field of the Invention
The present invention relates to position detection systems, and more particularly, to a three-axis motion tracking interferometer for measurement and correction of position errors between an article under test and a near-field measurement probe due to thermal drift.
2. Description of Related Art
High performance antennas are increasingly prevalent in the art as spacecraft, aircraft, ship and ground vehicle mission requirements become more sophisticated. One problem in the development and manufacture of high performance antennas is the accurate measurement of antenna performance. Traditionally, antenna performance measurement was conducted by placing the antenna at a remote location, and measuring the amplitude response characteristics as a function of orientation of the antenna in its operational range. Required measurement distances for high gain antennas range from fifty feet to three miles or more. This measurement technique, known as far-field testing, suffers from significant practical limitations, such as susceptibility to weather effects, ground reflections, and increasing real estate costs.
As an alternative to far-field testing, near-field testing was developed. A near-field test is conducted in an indoor test range using a probe to sample the field radiated near the antenna or article under test (AUT). An interferometer measures the interference between a pair of signals that have traveled through two separate paths, only one of which includes the AUT. The interferometer output is a measure of the complex gain (amplitude and phase response) of the AUT-probe transmission path relative to a reference path. A computer transforms the measured phase front data to the far-field angular equivalent using a Fourier transform technique, producing the desired antenna electrical property measurements.
The near-field measurement technique is also applicable to other types of reflecting bodies, emitters/receptors or transducers having other types of emitted waveforms, such as optical or acoustic waves, and is effective in measuring performance of antennas, lenses, anechoic chambers and compact ranges. The measuring probe may act as both a transmitting antenna and a receiving antenna for measuring a reflected phase front from a reflecting body. The reflecting or transducing bodies discussed herein are collectively referred to as antennas or transducers.
To make accurate near-field measurements, all the significant antenna energy must be sampled by the probe. Highly directive antennas, such as reflectors and waveguide phased arrays, send most of the energy in the forward direction normal to the antenna aperture. To test these types of antennas, a planar near-field scanner is utilized. Precision positioning systems, such as cartesian robots, are used to move the probe along a planar raster pattern approximately normal to the antenna aperture. Cylindrical and spherical scanners are also possible, in which the AUT is rotated relative to a measuring probe. To accurately reconstruct the measured field, the probe must sample points at some minimum spacing which is usually less than half the wavelength of the antenna signal (.lambda./2). Therefore, to achieve an accurate near-field measurement, the precise position of the probe and its planarity with respect to the AUT is critical.
Obtaining high accuracy position information for the probe relative to the test article has proven to be difficult to achieve. Undesired variations in the spacing between the probe and the AUT can be experienced due to thermally induced expansion and contraction resulting from periodic cycling of the air conditioning system within the test chamber. The temperature fluctuations affect the near-field measurement in two significant ways. First, the path length of the AUT-probe transmission path varies systematically during the near-field test. Second, the shape of the AUT mount and near-field scanner mount distorts adversely effecting azimuth and elevation of the AUT with respect to the probe. Each of these effects ultimately result in errors in the interference measurement.
Changes in path length are caused in part by thermal drift of the phase reference cable which carries the phase reference signal between the probe and the stationary RF measurement equipment. This path length change would be incorrectly interpreted in the near-field measurement as a change in distance in the z-axis direction between the probe and the AUT, which can significantly effect the interference measurement. Highly flexible cable, such as comprised of Goretex, is capable of maintaining electrical connection throughout the travel of the probe, but tends to experience the greatest amount of path length variation. For example, a one meter length Goretex phase reference cable that experiences a one degree Celsius temperature change at 12 GHz can have thermal drift of as much as 1.6 mils, which equates to a 0.58 degree antenna measurement error. Such an error would be outside of an acceptable measurement range for certain high performance antennas. Since near-field tests of large antenna arrays often utilize much greater phase reference cable lengths, even greater measurement errors can be experienced.
A second problem caused by temperature fluctuation is in thermal drift of the AUT mount and scanner mount. The temperature fluctuations can result in azimuthal and elevational distortion of the AUT position with respect to the probe. These distortions in AUT position further result in distortion of the energy emitted by the AUT, and decreased accuracy of the near-field measurement.
Two approaches have been utilized in the prior art to correct for the thermal expansion errors in the z-axis component of drift. The first method is to perform a single axis tie scan after completing the normal near-field raster scan. In a tie scan, the probe is moved linearly across the field of measurement, obtaining distance measurements in the z-axis direction for a plurality of points making up the linear tie scan. The tie scan measurements can then be used to correct the phase data obtained during the near-field raster scan. A disadvantage of this approach is that the points of the tie scan at the outer peripheral area of the field of measurement have reduced accuracy due to the low signal-to-noise ratio in this region, resulting in some corruption of the tie scan measurement data. Another disadvantage is that only the z-axis component of drift can be measured, and thermal drift in azimuth and elevation would not be detected.
A related method is similar to the tie scan, but utilizes individual linear points for repeated measurement of thermal drift. During the near-field raster scan operation, the probe is periodically returned to one of the points of the line to obtain phase correction data. To minimize scanner travel time, the probe is moved to the closest point of the line to the current probe position. However, this approach does not allow for azimuth or elevation measurements of thermal drift for the same reasons as the previously discussed method.
Thus, a critical need exists to provide an accurate measuring system for determining thermal drift between a probe and an article under test during a near-field measurement. It would be particularly desirable to obtain information concerning z-axis drift, as well as azimuth and elevation changes.